To endure for ever, i.e. to sit upon the throne for ever, as the next words explain it. This was accomplished only in Christ, the eternal King of the church, and of the world, who was of David's seed according to the flesh. As the days of heaven, i.e. for ever, as he now said; as long as the world shall have a being. It shall be as unchangeable and durable as the heavens themselves, which are of an incorruptible nature. See the like expression Deuteronomy 11:21 Jeremiah 31:35,36.
Related Commentaries of Psalms 89:29
Psalms 89:29
29 His seed also will I make to endure for ever, and his throne as the days of heaven.